(a) Technical Field
The present invention relates to a fuel cell stack with enhanced freeze-thaw durability. More particularly, it relates to a fuel cell stack that is designed to reduce contact resistance in a fuel cell to prevent water that is a by-product of an electrochemical reaction from being frozen under a sub-zero temperature condition when reactant gases such as hydrogen and oxygen gases are supplied to the fuel cell stack.
(b) Background Art
Polymer electrolyte membrane fuel cells (PEMFCs) have been widely used as a fuel cell for a vehicle. For a fuel cell stack, manufactured by stacking hundreds of unit cells of the polymer electrolyte membrane fuel cell, to be properly mounted in a vehicle, it is essential that it shows high power performance of at least tens of kilowatts (kW), and thus requires stable operation in a wide range of current density.
In a reaction for generating electricity in a fuel cell, after hydrogen supplied to the anode at which oxidation occurs in a membrane-electrode assembly (MEA) of the fuel cell is divided into hydrogen ions (protons) and electrons, hydrogen ions move to the cathode at which reduction occurs through a polymer electrolyte membrane, and electrons move to the cathode through an external circuit. Also, in the cathode, oxygen molecules, hydrogen ions, and electrons react with each other to generate electricity and heat and water as a by-product.
If a suitable amount of water is generated from the electrochemical reaction in the fuel cell, the generated water may serve to maintain suitable humidity conditions for the membrane-electrode assembly. However, if the amount of water generated is excessive, the excessive water may not be removed at a high current density, thus causing flooding of water throughout the cell. The flooding may prohibit reactant gases from being efficiently supplied to the fuel cell, thus deepening a voltage loss.
Water generates from the reaction between hydrogen and oxygen in the air in the polymer electrolyte membrane fuel cell. If the freeze-thaw cycle is repetitively changed from a sub-zero temperature to an ordinary temperature, components of the fuel cell and interfaces between the components such as an MEA and a gas diffusion layer (GDL) may be physically damaged thereby reducing its electrochemical performance and durability. Therefore, for the stable operation of a hydrogen fuel cell vehicle, it is crucial to increase the durability of a fuel cell stack under such a freeze-thaw cycle condition.
Various attempts have been conducted to increase the freeze-thaw durability of a typical fuel cell. For example, Korean Pat. No. 10-0802749, registered in 2008, discloses a technology of increasing the durability by optimizing a fuel cell cooling line structure to reduce the freeze-thaw cycle. U.S. Pat. Application Publication Nos. 2010/0143813 and 2008/0102326 disclose technologies of increasing freeze start capability by optimizing a method for controlling operation of a fuel cell. Also, U.S. Pat. Application Publication No. 2008/0241608 discloses a method of operating a fuel cell by removing ice generated at a sub-zero temperature by heat. However, these methods are too complex to apply in reality, and their effects are also limited. Accordingly, for a mass production of hydrogen fuel cell vehicles, it is necessary to develop a new technology to improve the freeze-thaw durability while at the same time making the implementation process as simple as possible.
As commercialization of fuel cells progresses, much research and development is being conducted on a gas diffusion layer (GDL) that is an essential component for managing water in a fuel cell. A GDL is attached to the outer surface of anode and cathode catalyst layers in an MEA of a fuel cell to perform various functions such as supply of reactant gases (hydrogen and oxygen gases in the air), transport of electrons generated from an electrochemical reaction, and minimize flooding in the fuel cell by discharging water generated from the reaction.
A GDL has been currently commercialized has a dual layer structure of a microporous layer (MPL) and a macro-porous substrate (or backing). The MPL has a pore size of less than about 1 μm when measured by a mercury intrusion method. The macro-porous substrate, on the other hand, has a pore size of about 1 μm to about 300 μm.
The MPL of the GDL may be manufactured by mixing carbon power such as acetylene black carbon and black pearls carbon with hydrophobic agent based on polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (PEP), and then may be coated on one or both surfaces of the macro-porous substrate according to applications. On the other hand, the macro-porous substrate of the GDL may be typically formed of carbon fiber and hydrophobic agents based on PTFE or PEP, and may include carbon fiber cloth, carbon fiber felt, and carbon fiber paper.
Since the GDL for the fuel cell has to be designed to have appropriate performance according to operation conditions and specific application fields of the fuel cell for, e.g., transportation, portable, and residential power generation, the GDL based on either carbon fiber felt or carbon fiber paper (in which overall characteristics such as supply of reactant gas, discharge of generated water, and compressibility/handling property for stack assembly are excellent), is more widely used for a fuel cell vehicles than carbon fiber cloth.
Also, a GDL has a significant influence on performance of a fuel cell according to various characteristics such as gas permeability, compressibility, degree of hydrophobicity of MPL and macro-porous substrate, structure of carbon fiber, porosity/pore distribution, tortuosity of pore, electrical resistance, and bending stiffness. Particularly, the GDL has a significant influence on the performance in the mass transport zone.
The gas diffusion layer needs to show excellent performance in a fuel cell, and have appropriate stiffness for excellent handling properties when hundreds of unit cells are assembled into a fuel cell stack. On the other hand, when the stiffness of the gas diffusion layer is too high in a direction of a roll, the gas diffusion layer is difficult to store in a roll form, thus reducing its mass-productivity capabilities.
Alternatively as noted above, when the stiffness of a gas diffusion layer 106 is deficient in a fuel cell, as shown in FIG. 1, the GDL 106 may intrude into a flow field channel 202 of a bipolar plate (also called as a separator) 200 (thus causing GDL intrusion). Thus, when the GDL 106 intrudes into the flow field channel 202 of the bipolar plate 200, a channel space for transferring materials such as reactant gases and generated water may not have enough room. Also, since the contact resistance between the GDL 106 and the rib (or land) 204 of the bipolar plate 200 and between the GDL 106 and an MEA 100 increases, the performance of the fuel cell may be considerably reduced.
Particularly, when the contact resistance in a cell increases, an interface between the GDL and the MEA or between the GDL and the bipolar plate may not be suitably maintained to generate an unnecessary gap. In this case, water generated in the fuel cell may be frozen to ice in the unnecessary gap under a freeze-thaw condition.
Thus, when there is ice generation, repetitive freeze-thaw cycles may damage the interface between components in the fuel cell. Accordingly, in order to increase the durability of a fuel cell, it is important to reduce the contact resistance so as not to generate a gap at the interface among the components of the fuel cell.
Generally, a bipolar plate for a fuel cell includes a major flow field and a minor flow field. Here, it is necessary for a GDL not to intrude into a channel of the major flow field. Therefore, it is important to increase the stiffness of the GDL oriented in the width (W) direction which is perpendicular to the major flow field direction of the bipolar plate than that oriented in the length (L) direction which is parallel with the major flow field direction of the bipolar plate (see FIGS. 2 and 3). Otherwise, as shown in FIG. 1, when a GDL having a low stiffness is arranged in the width direction of the major flow field of the bipolar plate, the GDL may further intrude into the major flow field channel of the bipolar plate. Accordingly, since a space in which ice may be generated at a sub-zero temperature (due to increase of damage or deformation of the interface in the fuel cell) increases, the freeze-thaw durability of the fuel cell may be reduced.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.